Vertical shaft wind turbines of this kind are previously known. However, if they are designed to deliver a large amount of power they become relatively fragile and so they are not suitable to be placed without supervision. On the other hand, if they are made robust to be able to be left without supervision their economic yield will become small.
In order to facilitate location without any supervision it is required for a wind turbine, besides control of rotation speed, e.g. when wind turbines drive an electrical generator, that the rotation speed of the wind turbine must be able to be limited upwards in order to prevent overspeeding at high wind forces, particularly when the turbine is unloaded, since then the turbine can be subjected to too high destructive centrifugal forces.
The limitation of rotation speed can take place by mechanical braking. A more sophisticated way to achieve limitation of rotation speed is to provide readjustment of the turbine blades for reducing the driving torque of the turbine. Thus, it is previously known in horizontal shaft turbines to provide, under influence of the centrifugal force and/or the wind force acting on a turbine blade, readjustment of the turbine blade in dependence upon the rotation speed of the turbine.
A proposal for a vertical shaft wind turbine having this function has been made by P. J. Musgrave in Proceedings of the International Symposium on Wind Energy Systems in Cambridge, Sept. 7-9, 1976. In this wind turbine the vertical turbine blades are unsymmetrically suspended in their longitudinal direction each on one single spoke. This suspension comprises a horizontal shaft which permits the turbine blades to pivot under influence of the centrifugal force acting on the turbine blades during rotation of the turbine. When the rotation speed increases, the turbine blades will pivot from a vertical position towards a horizontal position, with the result that the drive torque on the turbine emanating from the wind forces will be reduced, and in consequence the rotation speed will be limited.
However, in this wind turbine each turbine blade is supported in a single point of suspension which leads to great demands on the hinge construction. The hinge is permanently subjected to a varying load in response to the wind forces varying during the course of revolution. Moreover, the wind force often varies. In consequence of the continuously varying wind forces acting on the turbin blades there is a great risk for fatigue in the suspension hinge. Therefore this wind turbine is not suitable for location without any supervision. The construction for limiting rotation speed also does not facilitate aerodynamic braking of the turbine which is required if a start turbine, e.g. of the Savonius type, is employed.
Moreover, Mc Donnel Aircraft Company in a report by Robert V. Brulle in "Vertical-axis Wind Turbine Technology Workshop", May 17-20, 1976 has described a vertical shaft wind turbine, Giromill, having a robust construction where each turbine blade is supported by a least two spokes lying over each other. The turbine blades are individually readjustable around vertical shafts for bringing about an optimal angle of attack in each position of the turbine revolution, said readjustment being obtained by means of push rods, which abut a cam profile on the turbine shaft. However, no mechanism for the limitation of the rotation speed has been disclosed.